The present invention relates to machines for making shaped articles from synthetic thermoplastic material, and more particularly to apparatus for extruding tubular parisons (especially pendant parisons) which can be converted into pipes, bottles, vials, cans or other types of hollow articles, e.g., by resorting to a blow molding technique. Still more particularly, the invention relates to extruder heads.
The head of an extruder apparatus which furnishes parisons to one or more open-and-shut molds of a blow molding machine normally comprises an outer housing and an inner housing which latter supports a core in such a way that the core and the outer housing define an orifice for extrusion of a tubular parison. The core is secured to the inner housing by several radially disposed spokes so that the plastic material, which flows from a channel of the inner housing into an annular compartment between the outer housing and the core, must pass through the openings between neighboring spokes and is thereby divided into a plurality of discrete streams. The means for holding the core in requisite position with respect to the outer housing must withstand extremely high stresses ranging up to and in excess of 1,000 tons (depending on the size of the extruder head). In order to avoid streaking of the mass which advances toward the extrusion orifice, the material is normally caused to flow through two concentric chambers each of which contains a set of radial spokes, and the spokes of one set are angularly offset with respect to the spokes of the other set. If the extruder head is formed with a space for a supply of plastic material in front of an axially movable piston which can be pushed back by plastic material in the space, there remains very little room for installation of the core holder (as considered in the radial direction of the head). Consequently, and in view of the magnitude of forces acting on the core, the spokes (and hence the openings between neighboring spokes) are relatively long. This, in turn, causes the plastic material to offer a high resistance to the flow through such openings. The resulting pressure losses can amount to 120 atmospheres superatmospheric pressure. Another drawback of relatively long spokes and openings is that the period of dwell of plastic material in the openings is relatively long. Owing to elastic memory of plastic material (as a result of molecular orientation of thermoplastic material having a high molecular weight), the diameter of the parison invariably deviates from an optimum diameter.
Proposals to enlarge the diameter of the extruder head so as to provide room for several openings of relatively large cross-sectional area and for relatively bulky and strong spokes between the openings have met with limited success. Thus, if the extruder head is to furnish a small-diameter parison, the diameter of plastic mass which has advanced radially outwardly beyond the openings must be reduced before the mass reaches the extrusion orifice. Due to its elastic memory, the material of the parison expands as soon as it issues from the orifice so that the diameter of the expanded parison exceeds the desired diameter. Secondly, the manufacturing cost of extruder heads with relatively long openings between radial spokes is very high, especially since, owing to the high pressures involved, the component parts of the head must be made of high-quality material. The tolerances which can be achieved with available milling machines are unsatisfactory; therefore, the openings between the spokes must be formed by resorting to a costlier technique, e.g., to spark erosion. However, even such complex techniques cannot insure necessary uniformity in the dimensions of openings so that the pressure of material streams flowing through different openings varies within a wide range. Consequently, the rate at which the plastic material fills the space or spaces upstream of the extrusion orifice fluctuates with resulting adverse effect upon the quality of the parison. Still further, the parts of the core holder, especially of the ring between the two sets of spokes of a composite core holder, exhibit a pronounced elasticity in response to lateral stresses which, in turn, affects the configuration of the extrusion orifice so that the thickness of the parison wall varies, as considered in the circumferential direction of the orifice.
It was further proposed to utilize a short disk-shaped core which is inserted into a relatively short cupped outer housing and is held in position by several screws extending in parallelism with the axis of the extruder head. Such heads are used for the making of large-diameter parisons. The plastic material flows axially toward the central portion of the rear end of the core and the screws extend rearwardly from the core toward and into the housing (see German Utility Model No. 6,606,178) or vice versa (see German Utility Model No. 1,984,772). The just described extruder heads are not suited for the making of parisons which are to be introduced into the mold or molds of a blow molding machine, especially if the material of the parisons is a synthetic thermoplastic substance of high molecular weight and if the extruder head is to be formed with a reciprocable piston behind an annular material-accumulating space between the outer housing and the core.